1. Field of the Invention
The present invention relates to a method for producing a surface acoustic wave device, and more specifically, the present invention relates to a method for producing a surface acoustic wave device that stabilizes frequency response characteristics after frequency adjustment.
2. Description of the Related Art
Conventional surface acoustic wave devices include a piezoelectric substrate made of quartz and an IDT made of, for example, an Au thin film, Ta thin film or W thin film that is disposed on the piezoelectric substrate. These conventional SAW devices utilize surface acoustic waves mainly comprising Shear Horizontal or xe2x80x9cSHxe2x80x9d waves, as described in U.S. Pat. No. 5,847,486.
Such surface acoustic wave devices are manufactured via the following steps. In a first step, a wafer made of quartz is prepared. In a second step, a metallic film mainly containing, for example, Ta, Au or W is formed on an upper surface of the wafer by, for example, vapor deposition or sputtering. In a third step, unnecessary portions of the metallic film are removed by etching to form a multitude of patterns each including a plurality of interdigital transducers or xe2x80x9cIDTsxe2x80x9d and a plurality of reflectors. In a fourth step, each combination of the IDTs and the reflectors is considered as one surface acoustic wave element, and the wafer is cut at areas where the IDTs and reflectors are not located. In a fifth step, the cut and divided surface acoustic wave elements are housed in a package, and electrodes of the package and the IDTs are electrically connected via bonding wires. In a sixth step, the IDTs and reflectors of the surface acoustic wave elements are etched to adjust frequency response characteristics to desired values, and a lid is put on the package and the package is sealed.
However, the surfaces of, for example, the IDTs and reflectors or piezoelectric substrates are trimmed in such a frequency adjustment procedure, and the surfaces of these components are activated and become unstable. Accordingly, if these components are left standing in the air after frequency adjustment, the activated surfaces of the electrodes are altered or deteriorated to increase the masses of the electrodes to thereby decrease the frequency fo.
The variation in frequency due to alternation of the surfaces of the electrodes in the air is logarithmically proportional to time, and it is large immediately after frequency adjustment. Thereafter, the degree of alteration of the electrode surfaces gradually decreases, and, associated with this, the variation in frequency fo also tends to gradually decrease.
Accordingly, if a lid is put on the package and the package is sealed immediately after frequency adjustment, the package is sealed in a state where the frequency is rapidly varying. A time from frequency adjustment until sealing of the package is not constant, and the amounts of trimming upon frequency adjustment are not constant in individual surface acoustic wave devices. The frequency fo therefore largely varies from one surface acoustic wave device to another.
In addition, the development of variation in frequency after frequency adjustment does not stop even if the devices are left in the air or are simply subjected to a heat treatment, and the degree of variation in frequency after frequency adjustment cannot be predicted and an appropriate frequency adjustment cannot be performed.
In order to overcome the problems described above, preferred embodiments of the present invention provide a method for producing a surface acoustic wave device, which stabilizes the activated surfaces of electrodes such as IDTs after frequency adjustment, seals a device in a state where the frequency fo is stabilized, and finely and accurately controls the frequency of the device.
According to a first preferred embodiment of the present invention, a method for producing a surface acoustic wave device includes preparing the piezoelectric substrate, forming a thin film electrode on the piezoelectric substrate, etching thin film electrode for forming the IDT, trimming the piezoelectric substrate and the IDT for controlling frequency, and rapidly altering the surface of the IDT for stabilizing the surface of the IDT.
A method for producing a surface acoustic wave device according to another preferred embodiment includes a step of heating the IDT in an oxygen atmosphere or in air to oxidize a surface of the IDT during the step of stabilizing the surface of the IDT.
A method for producing a surface acoustic wave device according to another preferred embodiment of the present invention includes a step of heating the IDT in a nitrogen atmosphere to nitride the surface of the IDT in the step of stabilizing the surface of the IDT.
In another preferred embodiment of the present invention, a method for producing a surface acoustic wave device includes the step of finely controlling the frequency of the device by changing a heating temperature or heating time in the step for oxidizing or nitriding the surface of the IDT.
Other preferred embodiments may include the step of trimming the piezoelectric substrate and the IDT by physical collision of ions in the step for controlling the frequency.
According to various preferred embodiments of the present invention, the variation in frequency after frequency adjustment is minimized by oxidizing or nitriding the surface of IDT to thereby stabilize the surface of IDT.
As is described above, according to preferred embodiments of the present invention, an oxidized, nitrided, or another altered layer is rapidly formed on an activated surface of an electrode after frequency adjustment to thereby stabilize the characteristics of a surface acoustic wave device. Accordingly, the surface acoustic wave device can be packaged and sealed in a stable state with a less varied frequency, and precision after frequency adjustment is greatly improved.
In addition, the characteristics of the device are rapidly stabilized by a heat treatment, and a stable frequency is obtained in a short time, and a period of time until sealing of the package is greatly reduced. The time required for manufacturing the surface acoustic wave device is therefore greatly reduced.
Furthermore, the variation in frequency can be controlled by setting the temperature and time of heat treatment, and the frequency can be finely controlled after frequency adjustment procedure.
For the purpose of illustrating the present invention, there is shown in the drawings several forms that are presently preferred, it being understood, however, that the present invention is not limited to the precise arrangements and instrumentalities shown.